For many years cable has been laid underground. In recent times, many thousands of miles of optical cable have been laid to both replace electrical cable and as a new installation. While there are many ways to lay cable, it is often pushed or pulled through conduits. In view of the information explosion and the demand of the marketplace, it has been desirable and efficient to lay conduit in longer and longer lengths. Often, conduit is pulled and/or pushed through larger conduits or ducts for hundreds or thousands of feet at a time. Because of these lengths, tensile and other possible forces may build up to over 2000 lbs in the conduit while it is being pulled into place. In fact, it has been estimated that tensile forces acting on nominal 4 inch diameter duct may exceed 6000 lbs. while it is being pulled into larger conduits such as bored holes.
In order to withstand the tensile forces acting on the conduit, the conduit itself has to be strong. Moreover, because the conduit is often not long enough to be laid as a single piece it frequently has to be coupled to another length. Thus, the strength of the coupler and the connection have become a critical factors. Couplers for conduit are well known. For example, U.S. Pat. No. 6,179,347 B1, which is hereby included by reference, uses a type of coupler that is well known in the art. This coupler has a body which is cylindrical in shape and has an opening at both ends. On the circumference of the inner wall there is a rectangular groove. A conduit, which is designed to enter the coupler, has a complementary groove. When the grooves are aligned with each other they form a passageway for a flexible locking key. In the '347 B1 patent the locking key takes several forms. One of them is a series of complex and difficult to make V shaped elements that spring open. Another is a complex rhomboidal cross-section which would be critical to place in the right orientation.
Other embodiments of couplers are shown in U.S. Pat. No. 6,325,424 B1 and U.S. Pat. No. 6,352,288 B1, both of which are incorporated by reference. In addition to their complexity, they share a common problem with the other prior art couplers. None of them work well with high density polyethylene (HDPE) conduits. Most prior art couplers were designed to work with various materials, one of the most common being polyvinylchloride (PVC). However, HDPE is one of the more popular materials for conduit for economic, reliability and durability reasons. The traditional drawback of HDPE is that it is relatively soft. That is, it has a material modulus of elasticity typically less than 180,000 psi, whereas PVC has a modulus of elasticity of well over 350,000 psi.
When prior art couplers were connected to conduits made of the relatively soft HDPE, the conduits would bend or deform at the high tensile strengths endured during installation, use or removal. The result would be that the locking key would roll or move out of the grooves of the conduit or coupler and the connection would separate. Since the coupler was often far underground when the failure occurred, this was a major problem. The conduit had to be retrieved, another coupler put on the conduit and replaced or pulled back underground.
This invention provides a coupler that securely connects conduits made of HDPE, PVC and other commercially available materials so that they do not pull apart during the installation process, use and any subsequent removal. The coupler of this invention is particularly useful in connecting the ends of HDPE conduits and will successfully resist larger tensile forces. Other benefits of this coupler is a substantial savings in time and money in the laying of conduit since coupler failures will be minimized.